The present invention relates to ATM transmission systems for assembling and transmitting information into data cells having a predetermined bit number and, more particularly, to an ATM transmission system which assembles only the speech portion of a speech signal into cells for transmission.
To date, speech coding methods in speech coders have used a CELP (Code-Exited Linear Prediction) system which is one of speech analysis and synthesis methods, or an LD-CELP (Low-Delay Code-Exited Linear Prediction) system which is an improvement of the CELP system. The LD-CELP system is a typical vector quantization system in which successive excitation vector candidates from excitation vector codebooks are passed through a synthesis filter to transmit an index of a codebook with which decoded speech closest to the input signal is obtained. The index is a parameter to be transmitted. A problem encountered in such systems is that burying a voice/no-voice bit through bit steal of the index independently of the data coding process, results in different code vectors between the coding and decoding sides. That is, difficulty is encountered when burying the voice/no-voice bit in the coded data.
A prior art ATM speech transmission system utilizes such high efficient speech codec. A CLAD (Cell Assembling and Disassembling) apparatus for assembling and disassembling the transmission cells includes a speech detector for detecting voice/no-voice and the speech decoder with a high efficient coding of the speech signal. Only when the speech detector detects the voice, the coded data are assembled into cells and thus obtained cell data are transmitted without the addition of any code bit or the like directly by a mode switcher.
In a prior art ATM speech codec adopting the CELP system (as disclosed in Japanese Patent Laid-Open Publication Heisei 5-22153), the voice and no-voice are detected, and when the no-voice is detected the power supply is held "off" during the frame period, which is irrelevant to the coding, in order to save power. In this system, no voice/no-voice bit is multiplexed in the cell. In addition, it has been difficult in such a system to transmit the CELP coded data by multiplexing the voice/no-voice data, and therefore only the voice part of the signal is converted to cells without decoding the coded data. This gives rise to a problem if it is intended to introduce the ATM transmission line locally into an existing CBR (Constant Bit Rate) network constituted by CBR transmission lines. To this end, it is necessary that the coded data be decoded once and then coded again at the inter-connection point between CBR transmitter adopting the CELP coding system and the ATM transmitter. The speech quality, therefore, is deteriorated by repeated coding and decoding of the data. A further disadvantage is that for the repeated coding and decoding of data it is necessary to store once cells have a predetermined bit number, and this increases the propagation delay. A still further disadvantage is that in order to minimize the speech quality deterioration it is necessary to replace all the existing network CBR transmitters with ATM transmitters, which spoils the economy of the network.